I had this same problem today after having to copy my site over to an ssl folder on my server to make the https work.

My fix was easy. As others here have stated, the two configure.php files needed to be edited. The admin/includes/configure.php was changed to "define('ENABLE_SSL_CATALOG', 'true');" and the includes/configure.php was changed to "define('ENABLE_SSL', 'true');".

This needed to be done for both the html and the ssl folders on my server, meaning four files needed to be changed. My problem was that the changes were only made in the html folder and not the ssl folder. Make sure the files are set to writable in the file permissions before you edit or copy over new versions, or the changes might not take.